This invention relates generally to gas turbine engines, and, more specifically, to the compression modules therein, such as the booster and the compressor.
In a turbofan aircraft gas turbine engine, air is pressurized in a fan module and a compression module during operation. The air passing through the fan module is used for generating the bulk of the thrust needed for propelling an aircraft in flight. The air channeled through the compression module is mixed with fuel in a combustor and ignited, generating hot combustion gases which flow through turbine stages that extract energy therefrom for powering the fan and compressor rotors.
A typical compression module in a turbofan engine includes a multi stage booster which compresses the air to an intermediate pressure and passes it to a multistage axial flow compressor which further pressurizes the air sequentially to produce high pressure air for combustion. Both the booster and the compressor have rotor stages and stator stages. The booster rotor is typically driven by a low pressure turbine and the compressor rotor is driven by a high pressure turbine.
Fundamental in booster and compressor design is efficiency in compressing the air with sufficient stall margin over the entire flight envelope of operation from takeoff, cruise, and landing. However, compressor efficiency and stall margin are normally inversely related with increasing efficiency typically corresponding with a decrease in stall margin. The conflicting requirements of stall margin and efficiency are particularly demanding in high performance jet engines that require increased power extraction, while still requiring high a level of stall margin in conjunction with high compressor efficiency. In conventional designs, efficiency is usually sacrificed in order to achieve improved operability and increased stall margin.
Operability of a compression system in a gas turbine engine is traditionally represented on an operating map with inlet corrected flow rate along the X-axis and the pressure ratio on the Y-axis, such as for example, shown in FIG. 1 for a booster. In FIG. 1, operating line 102 and the stall line 101 are shown, along with several constant speed lines 104-108. Line 104 represents a lower speed line and line 105 represents a higher speed line as compared to the design speed line 103. As the booster is throttled from the operating line 102 at a constant speed, such as the design speed represented by the constant speed line 103, the inlet corrected flow rate decreases while the pressure ratio increases, and the booster operation moves closer to the stall line 101. In order to avoid a stall, the fans, boosters and compressors in a gas turbine engine are designed to have sufficient stall margin with respect to the stall line, such as line 101 shown in FIG. 1.
Maximizing efficiency of booster and compressor airfoils is primarily effected by optimizing the velocity distributions over the pressure and suction sides of the airfoil. However, efficiency is typically limited in conventional booster and compressor designs by the requirement for a suitable stall margin. Any further increase in efficiency results in a reduction in stall margin, and, conversely, further increase in stall margin results in decrease in efficiency.
High efficiency is typically obtained by minimizing the wetted surface area of the airfoils for a given stage to correspondingly reduce airfoil drag. This is typically achieved by reducing airfoil solidity or the density of airfoils around the circumference of a rotor disk, or by increasing airfoil aspect ratio of the chord to span lengths.
For a given rotor speed, this increase in efficiency reduces stall margin. To achieve high levels of stall margin, a higher than optimum level of solidity may be used, along with designing the airfoils at below optimum incidence angles. This reduces axial flow compressor efficiency.
Increased stall margin may also be obtained by increasing rotor speed, but this in turn reduces efficiency by increasing the airfoil Mach numbers, which increases airfoil drag. Obtaining adequate stall margin is a problem especially in the case of the booster. Boosters typically are run at relatively lower wheel-speeds, while at the same time, the throughflow velocity of the air is high. The booster is also unique in geometry because the air flowing through the rear stages of the booster is subjected to a significant change in direction of flow radially inward towards the longitudinal centerline axis. This results in a radial incidence swing imbalance as the booster is throttled to stall with large incidence swings in the hub region of the airfoils. In the booster, across the cruise and high power operating range where the booster bleed valve is closed, stall typically initiates in the hub region first, and therefore the incidence swings in the hub region are particularly detrimental to operability. The incidence swings in the hub region and the resulting stall margin loss become even more severe during engine operation when there is increased demand for auxiliary electric power from the high pressure spool in the engine. In conventional designs, efficiency is typically compromised to meet operability requirements.
It is, therefore, desired to further improve the stall margin of the boosters and other high through-flow/wheel-speed compressors without significantly sacrificing the efficiency for improving gas turbine engine booster and compressor performance.